Reducing friction between moving parts is a fundamental role of lubricants. This is especially significant, for example, in internal combustion engines and power transmission systems found in cars and trucks, in part because a substantial amount of the theoretical mileage lost from a gallon of fuel is traceable directly to friction. A variety of friction modifiers are widely known and used in such lubricants, including fatty acid esters and amides, esters of hydroxyalkyl acids, organo molybdenum compounds and the like.
Molybdenum friction modifiers are widely known and are effective over a broad temperature range, especially upon reaching temperatures of ˜120° C. or higher where chemical transformations form Mo-Sulfide glass coatings on surfaces. Molybdenum compounds however have some drawbacks, for example they can complex and interfere with dispersants and like other metal containing compounds, may suffer from particulate formation etc, as seen, for example, with the zinc anti-wear additive above. It is therefore desirable to reduce the amount of such friction modifiers in lubricants.
Fatty acid alkanolamides are known as both fuel additives and lubricant additives and have other uses in addition to friction reduction. For example, U.S. Pat. No. 4,729,769 discloses gasoline compositions containing reaction products of fatty acid esters and alkanolamines as carburetor detergents. The reaction products of mono- and di-alkanolamines of naturally occurring fatty acid derivative mixtures, e.g., the fatty acid glycerides in coconut oil, babasu oil, palm kernel oil, palm oil, olive oil, castor oil, peanut oil, rape oil, beef tallow oil, lard oil, whale blubber oil, and sunflower oil, are also disclosed. Useful amines include mono-ethanolamine, diethanolamine, propanolamine, isopropanolamine, dipropanolamine, di-isopropanolamine, butanolamines etc., although no products containing secondary hydroxyalkyl amines were prepared. The reaction product of diethanolamine with coconut oil is exemplified and preferred.
The amides of U.S. Pat. No. 4,729,769 are disclosed as friction modifiers for lubricants in US Pub Pat Appl 20040192565. As in U.S. Pat. No. 4,729,769 the product of diethanolamine with coconut oil is preferred in part because of the suggestion that mixtures of compounds which include transesterification products involving the hydroxyl group of dialkanolamine/dialkanolamide along with various glyceride side products may be beneficial in improving dispersibility of the amides. Primary hydroxyl groups as found in di-ethanolamine are disclosed as more reactive than secondary hydroxyalkyl amines as found in non-exemplified di-isopropanolamine.
Alkanolamides have an affinity for metal surfaces as found in, e.g., automotive engines, and are believed to form a film that adheres to these surfaces. The most effective friction modifiers form an even, protective surface coating at the metal-metal boundary where the surfaces contact each other, reducing the friction created by the interaction of moving engine parts. However, numerous challenges exist when designing additives that function in this environment without compromising or interfering with other processes or aspects of a smooth running engine.
A significant problem currently facing the development of organic friction modifiers is that while they must be polar enough to absorb on metal surfaces, they must also be soluble enough in the oil, for example, a non-polar mineral oil, so that they are completely solubilzed and not significantly self associated in the lubricant. Agglomerates of self associated compounds will not form the even film required on the metal surfaces for smooth operation of the engine. On the other hand, the compound must not be so soluble in the oil that it fails to come out of solution to coat the metal surfaces in a timely fashion.
U.S. Pat. No. 4,921,624 discloses alkanolamide lubricant additives similar to those of U.S. Pat. No. 4,729,769 and US Appl 20040192565, prepared by reacting a substantially saturated fatty acid triglyceride with a deficiency of dialkanolamine. Using less than one equivalent of amine per carboxy group leaves partially un-reacted mono, di- and tri-glycerides which help solvate the alkanolamides during use. As in the art cited above, products formed by reacting diethanolamine and coconut oil are exemplified. Unreacted glycerides and other reaction byproducts are believed to act as co-solvents and aid in forming stable oil solutions but the amount of the more active fatty acid amide is diluted.
Other attempts to prepare oil soluble alkanolamides include using unsaturated fatty acids in the preparation of the amide. Alkyl chains with unsaturation remove the linearity from the structure disrupting ordered the packing of crystal lattices, making self assembly of amide less likely, which helps keep the amide in solution. But inclusion of oxidizable unsaturates in the additive increases its likelihood of degradation while decreasing the stability of the overall oil formulation.
U.S. Pat. No. 4,512,903 provides lubricant compositions containing amides of hydroxy-substituted aliphatic acids and fatty amines. The use of long chain fatty amines is intended to improve the solubility of polar amide functionality in non-polar oils, however, this approach is often less effective in friction reduction as long non-polar polymer chains can make the molecule so strongly solvated that it does not readily form the desired film at the metal surface.
JP 06-074434 discloses a lubricating oil composition comprising diethanolamides of a C22-24 unsaturated acid which is said to be a better friction modifier than di-(hydroxyethanol) oleamide.
U.S. Pat. No. 4,280,915 discloses a water based drilling fluid which comprises an alkanolamide of a saturated C8-20 carboxylic acid and an alkanolamide of an unsaturated C18 carboxylic acid.
In the existing art, isopropanol amides and di-isopropanol amides are often disclosed but seldom exemplified. JP 10-008079A discloses a lubricating oil composition comprising an amide formed from a mono-alkanolamine and/or dialkanolamine with a C16-24 fatty acid as a detergent for reducing sludge. Di-isopropanol stearyl amide is exemplified as a single compound, however, and this product is a waxy solid.
There is a need for developing organic friction modifiers, preferably liquid, which are readily soluble in lubricating oils at ambient temperatures, i.e., room temperature, which form stable, storable oil formulations, but also readily organize to form a smooth film on a metal surface without negatively effecting the bulk performance of the lubricant.